1. Field of the Invention
This invention pertains generally to discrete cosine transform encoding and decoding of two dimensional images, and more particularly to a discrete cosine transform based method of removing block artifacts in the recovered image and increasing processing speed.
2. Description of the Background Art
Image processing plays an important role in applications such as electronic processing of photographic images, transmission of video images in communications systems, and in the storage, recovery and transmission of image data in general. In particular, discrete transform of finite-length data sequences plays an important role in the foregoing, as well as other, signal processing applications. As a result, various discrete transform methods have been developed. Of particular interest is the discrete cosine transform (DCT) which is generally recognized as the best and effective way to encode image information. For compression of a highly correlated image, DCT encoding approaches the optimum performance of the Karhunen Loeve Transform (KLT). Therefore, DCT has been adopted for use in the JPEG (Joint Photographic Expert Group) and MPEG (Motion Picture Expert Group) international data compression standards for still images and video images, respectively.
Although DCT is a highly effective method of encoding, in very low bit rate coding the DCT exhibits severe block artifacts which can affect resolution and, therefore, are often undesirable. Further, the amount of time required for the DCT computation can be excessive. For example, MPEG compression is an asymmetrical process, and takes considerably more time to compress an image than it does to decompress an image. An MPEG data stream contains two layers; a system layer that contains timing, video and audio synchronization and other demultiplexing information, and a compression layer that includes the actual audio and image streams.
Prior to encoding, if the incoming video source is an analog signal, it is sampled and converted to a digital data stream. Each frame of the image is pre-filtered, scaled to size, and edge-smoothed. In the encoding process, both interframe encoding (which compares adjacent frames) and intraframe encoding (which looks within a signle frame) schemes are used. For motion detection, MPEG uses the first frame as a reference frame, divides the visual array of the reference frame into macroblocks of 16.times.16 pixels, and compares the information in the macroblocks with subsequent frames. Each macroblock is compared with the surrounding 128.times.96 pixel area to eliminate temporal redundancies. This sets up the motion prediction of the images and reduces the data size of subsequent frames. After motion prediction, MPEG divides the macroblocks into four subblocks of 8.times.8 pixels for spatial compression within individual frames. The 8.times.8 block is transformed to frequency coefficients using the same DCT encoding used in the JPEG method, and the coefficients are quantized to remove unnecessary information. In this so-called "lossy" compression, most of the decimation takes place in the higher frequencies which are less perceptible to human eyes. Following DCT and quantization, the block is mapped to a one dimensional sequence using a zig-zag scan. Run-length encoding reduces redundant information by flagging long sequences of identical pixels. Still, however, the coding is a slow process due to the number of transform coefficients involved. Further, when the encoded information is subsequently decoded, since the DCT operated on 8.times.8 pixel subblocks of the image the recovered image can exhibit block artifacts where the block boundaries of the encoded image are visible.
While various methods have been developed for faster DCT computations, as well as for reducing block artifacts, there is no known method for both removing block artifacts and increasing the speed of the DCT computation, particularly with regard to low bit rate encoding. The present invention satisfies that need, as well as others, and overcomes deficiencies in encoding and decoding methods heretofore developed.